In a typical communication network, information is transported between network terminals by interconnecting links such as twisted pair metallic conductors, coaxial cables, fibre-optic cables, radio frequency wireless channels or over the air infra-red channels. The maximum distance over which information can be reliably transmitted in a network is mainly dictated by the type of interconnecting links used but is also dependent upon other factors such as the transmit power launched, transmission losses and the sensitivity of optical receivers used in the network terminals.
Where the distance between desired end points of a transmission exceeds the maximum distance over which information can be reliably transmitted, transit terminals such as repeaters and amplifiers are commonly used along the transmission path for signal amplification and regeneration.
Any data transmission between a transmitter and a receiver in a communication network is subject to unpredictable errors caused by signal degradation. It is well known that signals suffer degradation from a variety of sources such as, for example, noise, inter-symbol interference and distortion which are related to the nature of the transmission medium used. Signals may also be corrupted along the transmission path as a result of sampling and quantization. The degradation of a signal is expressed as a bit error rate (BER) which is the ratio between the number of erroneous bits counted at a particular point of interest in the network over the total number of bits received.
The extent of degradation of a particular signal may be directly measured using an eye closure diagram, which is the graphic pattern produced on an oscilloscope by the signal symbols superimposed over a single symbol interval. For a binary signal, such an eye diagram has a single eye which is open or closed to an extent determined by the signal degradation.
An eye closure diagram is useful for evaluating the transmission performance of a particular transmission link and can be used in a receiver for regenerating data. As is well known, the ability of a receiver to regenerate data received from a particular transmission link is dependent upon a threshold level or slicing threshold and a sampling phase. By monitoring the eye closure of the incoming signal, the optimum slicing threshold and sampling phase for the receiver can be determined more easily.
U.S. Pat. No. 4,823,360 entitled "Binary data regenerator with adaptive threshold level" which issued on Apr. 18, 1989 to Tremblay et al. (hereinafter referred to the "360 patent"), discloses a data regenerator which dynamically monitors the eye closure of an incoming data signal for regeneration. In order to monitor the data signal eye closure, the data regenerator disclosed in the 360 patent uses two slicing thresholds within the data eye (hereinafter referred to as "reference slicing thresholds") for producing "pseudo-errors" at a preset BER on binary ones and zeros of the data signal. The pseudo-errors do not appear on the in-service data path and as a result, do not affect the data regeneration. The thresholds are dynamically adjusted so that the number of pseudo-errors generated on binary ones and zeros is maintained to correspond to the preset BER. On the in-service data path, the data regenerator operates to set a third slicing threshold level (hereinafter the "data slicing threshold") optimally within the data eye in relation to the reference slicing thresholds and move the placement of this threshold as a function of time within the eye to produce an optimum slicing level at an optimum sampling phase. The technique disclosed in the 360 patent provides some measure of the transmission link performance as a function of slicing level and sampling phase and is particularly useful to regenerate incoming data with an optimally placed slicing threshold.
However, a disadvantage of this approach is that the optimization of the data slicing threshold and sampling phase is carried out on the in-service data path while the incoming data is being regenerated. By optimizing the data slicing threshold and sampling phase on the in-service data path, errors can be introduced in the data signal thereby effectively increasing the BER of the transmission system and reducing performance. For low bit rate signals where the optimum sampling phase can be located near an edge of the data eye, this can lead to a failure of the data path.
Another data regenerator which optimizes the data slicing threshold and sampling phase on the in-service data path during data regeneration is disclosed in U.S. Pat. No. 5,896,391 entitled "Forward error correction assisted receiver optimization" which issued on Apr. 20, 1999 to Solheim, et al. (hereinafter referred to the "391 patent"). Instead of using reference slicing thresholds and pseudo-errors to monitor the eye closure of the incoming signal and optimize the slicing threshold and sampling phase, the data regenerator disclosed in the 391 patent prepares BER maps of the incoming data signal based on actual errors present in the incoming data signal as they are detected by error detection circuitry.
It is well known that errors present in an incoming data signal transmitted over a transmission link may be detected (and corrected) at a receiver by using error correction codes such as a forward error correction (FEC) codes. The error correction codes are typically embedded in the data signal at the transmitting site before transmission. At the receiver, the errors present in the regenerated data signal can be detected and corrected by comparing the codes received with the known codes transmitted.
Monitoring the eye closure of an incoming data signal based on actual errors detected in the regenerated data signal such as is disclosed in the 391 patent might be appropriate in proprietary systems where error detection techniques are already used. However, this technique would not be appropriate in systems which do not use any error detection techniques. This would certainly be true of low data rate systems in the order of 2.5 Gb/s where error detection is not required. Further, as there is presently a strong push for protocol and bit-rate independent systems, it would be desirable to be able to monitor the eye closure for data regeneration independently of any proprietary information.
The operation of the data regenerator disclosed in the 391 patent is divided into an error mapping mode, an optimization mode and a data regeneration mode. More specifically, the data regenerator functions in the error mapping mode to monitor the eye closure by preparing a BER map of the incoming data signal. The data generator can also operate in the optimization mode to determine based on the BER map prepared, an optimum data slicing threshold and sampling phase placement within the data eye. With the slicing threshold and sampling phase optimized, the data regenerator can operate in a data regeneration mode to regenerate the incoming data signal.
Although the data regenerator disclosed by the 391 patent operates in separate modes for BER mapping, optimization of the slicing threshold and sampling phase, and data regeneration, actual errors are nevertheless generated on the in-service data path during the BER mapping and optimization process. The data regenerator disclosed therein would be appropriate for proprietary data signals with proprietary data rates which do not require frequent BER mapping and optimization updates as the eye closure in such cases remains essentially the same.
However, in situations where the eye closure continuously changes requiring frequent BER map and optimization updates, the data regenerator may introduce a substantial number of errors in the regenerated data signal which may not be all correctable by standard error correction techniques. This would occur for example where the transmission link is used for data signals operating at different bit rates or operating with different transmission protocols. Again, as network technology evolves toward multi rate and protocol independent systems, it would be desirable for data regeneration to be able to continuously monitor the eye closure independently of any rate or protocol change without introducing any errors into the regenerated data.